Rebel groups threatened to abandon the ceasefire if the government continued its assault on areas under their control.

Rebel leaders told Al Jazeera that Assad's government were trying to take advantage of the situation to further expand its territory.

At least 10 air strikes hit rebel-held villages and towns in the strategically important Wadi Barada valley near Damascus, activists said.

Delicate moment for truce

Al Jazeera's Hashem Ahelbarra, reporting from Turkey's Gaziantep near the Syria border, said it was a delicate moment for the ceasefire.

"They [Syrian rebel groups] have sent an urgent appeal to the UN and to Turkey, which is the key player in the ceasefire, to negotiate with the Russians and try to stop the Syria government, warning that if this continues, there will be no option but to resume the fighting," he said.

"The terms of the ceasefire insist that the moment it comes into effect, there should be no military operation, no party should take advantage of it. But the Syrian opposition would need that guarantee, that the guns must fall silent across Syria."

The Syrian civil war started as a largely unarmed uprising against Assad in March 2011, but quickly developed into a full-on armed conflict.

Calculating a precise death toll is difficult, partially owing to the forced disappearances of tens of thousands of Syrians whose fates remain unknown.

Almost 11 million Syrians - half the country's prewar population - have been displaced from their homes.